A flight plan is a detailed description of the route that an aircraft is to follow as part of a planned flight. The flight plan is currently managed onboard civilian aircraft by a system referred to as a flight management system, which will be referred to hereinafter as FMS and which makes the course that is to be followed available to the flight crew and other on-board systems. These systems amongst other things provide navigation aid by displaying useful information to the pilots, or by communicating flight parameters to an automatic pilot system. FIG. 1 shows a block diagram illustrating the structure of a known FMS of the prior art. An FMS-type system 10 has a man-machine interface 12 comprising, for example, a keyboard and a display screen, or quite simply a touch-sensitive display screen, as well as at least the following functions which are described in the ARINC 702 standard:                Navigation (LOCNAV) 101, for optimally locating the aircraft using geographic-location means 130 such as satellite navigation or GPS, GALILEO, VHF radio navigation beacons and inertial units. This module communicates with the aforementioned geographical-location devices;        Flight plan (FPLN) 102, for the acquisition of the geographical elements that make up the framework of the course to be followed, such as the fixes imposed by the departure and arrival procedures, waypoints, or aerial corridors more commonly referred to as airways;        Navigation database (NAVDB) 103, to build the geographical courses and procedures from data included in databases relating to the fixes, beacons, intercept or altitude legs, etc;        Performance database (PRFDB) 104, containing aerodynamic and engine parameters relating to the craft;        Lateral trajectory (TRAJ) 105, for constructing a continuous trajectory from the points in the flight plan, consistent with the performance of the aircraft and complying with accuracy requirements (RNP);        Predictions (PRED) 106, to construct a vertical profile that is optimized on the lateral and vertical trajectory. The functions that form the subject matter of the present invention affect this part of the computer;        Guidance (GUID) 107, for guiding the aircraft on its three-dimensional trajectory in the lateral and vertical planes while at the same time optimizing aircraft speed. In an aircraft equipped with an automatic pilot device 11, the latter can exchange information with the guidance module 107;        Digital data link (DATALINK) 108 for communicating with the control centres and other aircraft 13.        
The flight plan is input by the pilot, or alternatively via a data link, from data contained in the navigation database.
The pilot then inputs the aircraft parameters: mass, flight plan, range of cruising altitudes, and one or a plurality of optimization criteria such as the CI. These inputs allow the TRAJ module 105 and PRED module 106 respectively to calculate the lateral trajectory and the vertical profile, i.e. the flight profile in terms of altitude and speed, which for example minimizes the optimization criterion.
The flight plan managed by the FMS is coded in a specific way in the form of a series of legs which are defined by an aeronautical standard. In commercial aviation, the international standard ARINC 424 defines various types of legs. The standard uses the widely-understood English terminology “legs”.
The legs in the ARINC 424 standard are described in greater detail hereinbelow. A flight plan is made up of an ordered series of legs, a leg corresponding to an instruction for the FMS to follow in order to calculate the trajectory of the aircraft. Each leg generates a trajectory portion or elementary trajectory. This elementary trajectory corresponds to a geometric element that may be a straight-line segment, an arc or combinations of straight-line and arc segments, but the term “segment” must not in any way be interpreted here in its geometric sense. The lateral trajectory is calculated from one leg to another in accordance with a certain number of conventions.
A flight plan is produced by stringing together, amongst other things, procedures that are stored in the navigation database 130 and structured in accordance with the aforementioned standard ARINC 424. These procedures, made up of a set of legs, come from data supplied by the countries concerned, corresponding to the points and procedures in force in the airspace being crossed. For example, in order to construct a flight plan, the pilot chooses various procedures indexed by a name. The FMS then extracts these procedures from the navigation database then strings the successive procedures together (the English expression “stringing” is commonplace) to generate the flight plan. The method of stringing according to the prior art is described later on.
The aeronautical standard ARINC 424 defines a set of leg types, each type having its own characteristics referred to as attributes corresponding to a type of data needed for calculating the elementary trajectory corresponding to the type, for example instructions to be followed in terms of position, altitude, heading or course.
The international standard ARINC 424 defines a set of 23 leg types. The standard ARINC 424 also defines all of the combinations in which these legs can be strung together, notably excluding certain sequences. The legs currently defined in the ARINC 424 standard are listed in the table below.
Thus, the ARINC 424 standard defines:                8 types of legs known as fixed legs, the start or termination of which is a fixed waypoint on land, published in terms of latitude and longitude (a navigation fix or “waypoint” usually denoted by the abbreviation WPT).These are legs of the IF, CF, DF, TF, AF, RF, FC, FD type.        11 types of legs referred to as “floating” legs, the termination of which consists in the achievement of a variable condition, such as, for example, legs which terminate when the aircraft has reached a certain altitude. These are legs of the VA, CA, FA, VI, CI, VD, CD, VR, CR, VM, FM type. Certain floating legs, FA, FM, CD and CR may, however, use a fixed waypoint.        3 types of leg referred to as “holding legs” which correspond to racetrack circuits. These are the legs of the HM, HA, HF type.        one type of leg referred to as “course reversal” which corresponds to a procedure turn away from and then back towards the trajectory. These are legs of the PI type.        
TypeARINC 424 NameMeaningIFInitial FixInitial fixed point on the groundCFCourse To a FixGo to/follow a course to a fixed point (fix)DFDirect to a FixGo directly (in a straight line) to a fixTFTrack between two FixesFollow a great circle route between 2 fixesAFArc DME to a FixDefines an arc of a circle about a DME beacon at a specifieddistance with a limit on angle.RFRadius to a FixDefines an arc of a circle between 2 fixed points (the 1st pointbeing the fix of the pervious leg), on a centre fix.VIHeading to InterceptDefines a heading to intercept the next legCICourse to InterceptDefines a course to intercept the next legVAHeading to AltitudeDefines a heading to be followed until a specific altitude isreachedCACourse to AltitudeDefines a course to be followed to a specific altitudeFAFix to AltitudeDefines a course to be followed, from a fix to a specified altitudeVDHeading to DMEDefines a heading to be followed to intercept a specified DME arcDistanceCDCourse to DME DistanceDefines a course to be followed to intercept a specified DME arcVRHeading to RadialDefines a heading to be followed to intercept a specified radialCRCourse to RadialDefines a course to be followed to intercept a specified radialFCTrack from Fix toDefines a course to be followed from a fix for a specified distanceDistanceFDTrack from Fix to DMEDefines a course to be followed from a fix to intercept a DME arcDistance(specified DME distance)VMHeading to ManualDefines a heading without a termination (infinite half straight line)FMFix to ManualDefines a route, from a fix, without a termination (infinite halfstraight line)HAHippodrome to AltitudeRacetrack circuit, with hold to an altitudeTerminationHFHippodrome to FixRacetrack circuit with one single turnTerminationHMHippodrome to ManualManually terminated racetrack circuit, no exit conditionTerminationPIFix to ManualProcedure turn defined by a route away from the fix, followed bya one hundred and eighty-degree course reversal to intercept theinitial outbound course for the return.